Any Wireless Local Area Network (WLAN) device that supports an Institute of Electrical and Electronics Engineers (IEEE) 802.11 Standard (e.g., IEEE Std. 802.11-1997, 802.11a, 802.11e, etc.) includes two main parts: 1) a physical (PHY) layer signaling control device; and 2) a medium access control (MAC) device. The function of the PHY device is to transfer data packets over the air interface. Among other things, the function of the MAC device is to fairly control access to the shared air interface.
The minimal MAC protocol consists of two frames: 1) a frame sent from a transmitter to a receiver; and 2) an acknowledgement (ACK) from the receiver that the frame was received correctly. If a transmitter has multiple packets to send to the receiver, some versions of the 802.11 Standard require the transmitter to wait for an ACK after transmission of each packet. In addition, the transmitter must wait for a particular time interval, referred to as the Interframe Space (IFS), after receiving the ACK and before transmitting the next packet.
Other versions of the 802.11 Standard (e.g., IEEE Std. 802.11e) support transmission of packets with selective acknowledgement. This feature is referred to as “Block ACK.” The Block ACK feature enables the transmitter to send the next packet to the same receiver without necessarily waiting for an ACK. Instead, after negotiating for access to the air interface, the transmitter sends the first packet, waits an inter-frame space (IFS) after the end of the first packet, and sends the next packet. After the transmitter has sent all of its packets to the receiver, the transmitter asks the receiver for a response, which indicates an ACK for all of the previously transmitted packets.
The Block ACK (BA) mechanism provided by the 802.11e Standard is referred to herein as the “immediate BA mechanism.” The immediate BA mechanism provided by the 802.11e Standard is essential for realizing the throughput gains expected with 802.11n. However, the existing protocol imposes large memory requirements on devices that support this optional feature. The immediate BA protocol allows an originator to send a block data frames to a recipient and then request an acknowledgement for the block. To do this, the recipient maintains a scoreboard with a starting sequence number (SSN) and a bit marking each received data frame per each of established BA agreements. The process is under the control of the originator, which decides when the data frames are sent and when the block acknowledgement request (BAR) is sent to solicit a BA.